In operation of an electronic device, it is often necessary to boost a lower voltage level in order to power circuits requiring a higher voltage level. This boosting function is usually accomplished by a boost converter.
Referring to FIG. 1, a conventional boost converter is shown. The boost converter includes a direct current (DC) voltage input terminal Vin, a DC voltage output terminal Vout, a fuse F101, a first inductor L101, a second inductor L102, a switching power transistor Q101, a rectifying diode D101 and a filtering capacitor C102. The second inductor L102 and the first inductor L101 are coupling inductors, with a turn ratio of the second inductor L102 to the first inductor L101 being N.
The DC voltage input terminal Vin is connected to an anode of the rectifying diode D101 via the fuse F101, the first inductor L101 and the second inductor L102 in sequence. A cathode of the rectifying diode D101 is connected to the DC voltage output terminal Vout. The switching power transistor Q101 has a drain connected between the first inductor and the second inductor, a source connected to the ground GND, and a gate for receiving a drive signal DRV. The filtering capacitor C102 has one terminal thereof connected to the ground GND and the other terminal thereof connected to the DC voltage output terminal Vout.
The boost converter operates on the following principle.
When the switching power transistor Q101 is turned on, the first inductor L101 is charged by the DC voltage input terminal Vin to produce induced electromotive forces in the first inductor L101 and the second inductor L102. A voltage difference across the first inductor L101 is Vin, and a voltage difference across the second inductor L102 is N*Vin. Because the left terminal of the second inductor L102 is grounded, the voltage at the right terminal of the second inductor L102 is −N*Vin, i.e., the anode voltage of the rectifying diode D101 is −N*Vin.
When the switching power transistor Q101 is turned off, the first inductor L101 begins to be discharged and the voltage at the DC voltage output terminal Vout reaches a value of Vo, i.e., the cathode voltage of the rectifying diode D101 is Vo.
As can be known from the above description, the maximum voltage difference between the cathode and the anode of the rectifying diode D101 is equal to Vo+N*Vin, so the rectifying diode D101 must be able to withstand a voltage higher than Vo+N*Vin. Therefore, this boost converter has a high requirement on the voltage withstand capability of the rectifying diode D101, which imposes a limitation on choice of the rectifying diode D101.
Additionally, when the switching power transistor Q101 is turned on, the rectifying diode D101 is turned off because the voltage at the left terminal of the second inductor L102 is higher than that of the right terminal. In this case, the second inductor L102 cannot serve the function of storing energy because no current loop is formed therethrough. This makes the utilization factor of the inductor relatively low.
Accordingly, an urgent need exists in the art to provide a boost converter which can eliminate the limitation on choice of the components and can increase the utilization factor of the coupling inductors.